Catalytic converters are being used to treat exhaust gases developed from the burning of a hydrocarbon fuel in an automotive internal combustion engine. One of the functions of these catalytic converters is to reduce the oxides of nitrogen produced in the combustion process. In this reduction reaction, it is possible to form ammonia as an end product. This ammonia, in some cases, is re-oxidized on an oxidation catalyst to form, once again, the oxides of nitrogen which had been sought to be eliminated.
Those skilled in the art have attempted to remove oxides of nitrogen from the gas stream evolved from an internal combustion engine in two general manners. In a first manner, a nitrogen oxide reduction catalyst is used by itself and the gases passing therethrough are generally overall reducing in composition. By reducing, it is meant that the gases have less oxygen present than fuel to be burned. This type of catalyst system is designed solely to eliminate oxides of nitrogen.
A second approach to elimination of oxides of nitrogen has been the so-called three-way catalytic converter. This type of converter operates at or near a stoichiometric air/fuel ratio in which the air present is sufficient to burn the fuel present. In this type of a converter, unburned hydrocarbons and carbon monoxide are oxidized and oxides of nitrogen are reduced.
Those skilled in the art are aware that it is possible to get ammonia produced as one of the final reaction products when either a reduction catalyst or a three-way catalyst is used to treat exhaust gases. It is a principal object of this invention to provide a method and a catalyst for treating exhaust gases from an internal combustion engine which suppresses the production of ammonia.
U.S. Pat. No. 4,061,713 is directed to a catalyst in which a catalyst system comprises molybdenum, rhodium and, optionally, platinum supported on a suitable support media. In accordance with the teachings of this patent, the molybdenum, rhodium and optional platinum content of the catalyst system may be from about 0.01 to 0.1% by weight of each metal and the catalyst system preferably contains, by weight based on the weight of the catalyst support media, about 0.02 to 0.08% molybdenum, about 0.02 to 0.04% rhodium, and about 0.04 to 0.08% platinum, the atomic ratio of molybdenum to rhodium being preferably from about 1:1 to 4:1.